This invention relates to an internal expansion-type drum brake and more particularly, to a drum brake of the type which comprises a rotary brake drum, a pair of arcuate brake shoes carrying friction linings thereon for engaging the inner surface of said drum and pivoted to a stationary backing plate, a mechanical actuator positioned between one end of said brake shoes to pivot the brake shoes away from each other so as to cause the friction linings to engage the inner surface of the drum to thereby brake the drum when said actuator is operated, a rockable lever having one end connected to one of said brake shoes and the other end connected to a mechanical operation means such as non-extensible flexible cable and the like and an automatic friction lining wear compensation means acting between an intermediate position between the opposite ends of said lever and the other brake shoe to automatically adjust the clearance between the drum and brake shoes.
In the drum brake of the type referred to herein-above, it is necessary to remove the drum to replace the brake shoe or shoes when the friction lining on at least one of the brake shoes is badly worn. However, frequently the shoe or shoes do not retract from the drum for the reason that the automatic brake clearance adjusting means is present or the adjusting means is not accessible from outside whereby the removal of the drum is made difficult.
In order to eliminate this problem, a drum brake has been proposed in which a lever cooperates with a stop member (a lever retraction limit member) adapted to determine the brake clearance between the brake drum and brake shoes and is held by the stop member in a predetermined first position on this side of and spaced from the fully retracted position and the stop member is operated from outside to move the lever from said first position to said fully retracted position whereby the brake clearance can be increased and the removal of the drum is made easy.
For better understanding of the features of the present invention, reference will be made first to FIGS. 1 and 2 of the accompanying drawings which show a conventional drum brake. When the conventional drum brake is applied under normal conditions, a hydraulically operated cylinder-piston assembly 5 is operated under liquid pressure supplied from a brake liquid source (not shown) and acts on one end face of each of a pair of arcuate brake shoes 1, 2 pivoted to a backing plate 4 so as to pivot the brake shoes away from each other to cause the linings on the shoes to engage the drum, and an abutment 6 secured to the backing plate 4 is abutted by the other end face of the brake shoes 1, 2 to receive the torque applied to the brake shoes.
When the drum brake is in its non-operative position, the brake shoes 1, 2 are maintained in their disengaging position with respect to the drum by return springs 7, 8 which extend between and are anchored to the webs of the brake shoes 1, 2.
The drum brake is also operated as an emergency or parking brake by a mechanical actuator. The mechanical actuator has a lever 9 having one end pivoted to the web of the trailing brake shoe 1 by means of a pivot pin 10 in a position adjacent to the cylinder-piston assembly 5 which serves as the hydraulic actuator. The other or free end of the lever 9 is connected to one end of a non-extensible and flexible cable 11 the other end of which extends through an opening 12 in the backing plate 4 into the brake. The lever 9 acts in an intermediate position between the opposite ends thereof on one end of a strut 13 the other end of which engages the web of the leading brake shoe 2. The opposite ends of the strut 13 are bifurcated as shown by 15 to engage the lever 9 and a groove 16 in the leading brake shoe 2, respectively.
The lever 9 is normally held in a position on this side of and spaced from its fully retracted position by means of a deformable stop member 17 formed of synthetic resin and extending through an opening 18 in the backing plate 4 into the brake. The inner end of the stop member 17 engages an edge of an abutment 19 which is integrally formed with the lever 9 and is disposed at right angles to the plane of the lever 9. The abutment 19 abuts against the inner surface of a boss 20 at the free end of the flexible cable 11.
With the stop member 17 mounted on the brake, the leading end face of the stop member engages the abutment 19 to position the lever 9 in a first partially retracted position offset inwardly from its fully retracted position against the force of the return springs 7, 8.
When it is desired to remove the drum from the brake, the stop member 17 is first taken out of the backing plate 4 whereby the lever 9 is allowed to pivot freely about the pivot pin 10 outwardly to the fully retracted position under the action of the springs 7, 8 and thus, the clearance between the drum and friction linings 3 on the brake shoes 1, 2 is increased. The construction and arrangement of the conventional drum brake is as mentioned hereinabove.
That is, in the conventional internal expansion-type drum brake in which the pair of arcuate brake shoes 1, 2 carrying the friction linings 3, 3 thereon for engaging the rotary drum are pivoted to the stationary backing plate 4 and adapted to be separated at one end from each other, when the brake is operated by the mechanical actuator connection including the angularly movable lever 9 one end of which is pivoted to the trailing brake shoe 1 and the other end of which is connected to the brake operation means such as the non-extensible flexible cable 11, the strut 13 is provided to act between an intermediate position between the opposite ends of the lever 9 and the leading shoe 2 and an automatic clearance adjusting mechanism 14 is provided to automatically adjust the clearance between the brake drum and brake shoes 1, 2 in order to compensate for the wear of the friction linings 3, 3 on the brake shoes, characterized in that the lever retraction limit stop member 17 is mounted on the backing plate 4 to engage the lever 9 so as to determine the clearance between the drum and shoes when the brake is in its non-operative position, the engaging between the stop member 17 and lever 9 serves to normally hold the lever in the first partially retracted position offset inwardly from the fully retracted position, and the lever retraction limit stop member 17 is adapted to be manipulated from outside of the brake to allow the lever 9 to move from the first retracted position to the fully retracted position to thereby increase the clearance between the drum and shoes.
Thus, the conventional drum brake referred to hereinabove has succeeded in simplifying the removal of the rotary drum from the brake. In the conventional drum removal means, the stop member 17 is of an integral construction formed of any suitable deformable synthetic resin and in consequence, in order to secure the stop member to the backing plate 4 in a stabilized condition, the stop member has to be necessarily formed with a wide margin for fastening or formed in a large size. Therefore, such conventional drum removal means has the disadvantages that the stop member 17 can not be easily incorporated into the backing plate 4 and that, once removed from the backing plate 4, the stop member can not be used again because of deformation and/or breakage of the stop member or the margin of the opening to which the stop member is fastened during the removal operation of the stop member.